(1) [The Spread of IP Networks/IP Telephones]
In recent years, Internet connection services which use ADSL/FTTH have spread in homes and small- to medium-sized offices. Also, in companies and Universities, etc., communization of information processing equipment by LAN and a transition from conventional private lines to VPN (Virtual Private Network: technology to create a private line over the Internet), etc., are planned. Behind such technology, IP networks and IP telephones that use the IP networks are becoming widespread. The IP telephones use technology to send a voice as IP packets (VoIP: Voice over Internet Protocol). Because the IP telephones can execute voice calling over a cheap IP network/router without using expensive equipment such as a switching equipment, a large cost merit is being brought to users who make frequent long-distance calls.
(2) [The Increase of Facsimile Communication Over an IP Network]
As a background to the development and spread of the IP networks and IP telephones, in homes and of course also in companies, the number of facsimile terminals connected to an IP network is increasing, and facsimile communication over an IP network is increasing.
FIG. 1 is a diagram describing a connection between a general public switched telephone network, the Internet, and an IP network/IP telephone.
A communication path 100 indicates facsimile communication which traverses only a telephone station and a public switched telephone network. A communication path 101 connects a facsimile 103 with an ISP (Internet Service Provider) 108 via a VoIP router 107, and indicates a communication path which executes facsimile communication which passes through the Internet 109. Also, a facsimile 105 is connected to an IP network 111 such as a company private line or a VPN, etc., via a VoIP router 110, and it is possible to execute facsimile communication connecting to the public switched telephone network 113 through a VoIP gateway 112 (communication path 104). Also, a communication path 106 indicates transmission between facsimiles connected in the IP network 111. With these various connection configurations, facsimile communication through the IP network 111 can be realized.
(3) [Problems with Facsimile Communication Through an IP Network]
In facsimile communication over a conventional public switched telephone network, interruption of communication rarely occurred because the necessary band for communication over the transfer path was guaranteed. However, as facsimile communication over an IP network increases with the spread of IP network, these communication interruptions during facsimile communication will gradually increase.
In facsimile communication through an IP network, facsimile signals are divided into packets of audio signals and transferred, and on the receiving side, those packets are reassembled and the facsimile signal is obtained by assembling and decoding the audio signal. Because there is no guarantee of communication time in the IP network, and also because when the IP network is congested a band for an audio signal and a band for other data communication cannot sufficiently be taken, a line interruption due to loss of a frequency band or packet delay/loss can occur. With high communication volume, particularly when clumps occur during time periods when data communication is concentrated, such a line interruption can easily occur in an internal company network and the like.
(4) [The Cause for Degradation of Audio Quality in an IP Network]
Before an audio signal from a sending device arrives in a receiving device, A/D conversion, encoding, and packetizing are executed on the signal in the VoIP router (or the VoIP gateway between the public switched phone network and the IP network). Then, in the router in the IP network, routing relay/transfer is executed, and the audio signal is restored by executing packet assembly, decoding and D/A conversion in the VoIP router (or VoIP gateway) on the receiving side.
At this time, the communication speed of packets transferred in the IP network is not constant. For this reason, for a plurality of transferred packets, there is a difference in the timing at which each packet arrives at the VoIP on the receiving side due to network congestion and interruption of the IP network.
FIG. 2 is a diagram illustrating an example of differences in packet arrival timing and packet loss during communication over an IP network.
FIG. 2 illustrates an example of packets sent from a facsimile 201 at constant intervals through a VoIP router being received by a facsimile 202 over an IP network 203. In this manner, if the difference in arrival timing of each packet is large and packet loss occurs in the IP network 203, a phenomenon that a signal wave sampled by the VoIP router on the sending side cannot be restored occurs. This has the effect of instantaneous interruption (temporary loss of audio signal) in the audio signal played back based on the received packets.
(5) [Effects of Degradation of Audio Quality on Facsimile Communication]
Instantaneous interruption of an audio signal caused by a difference in packet arrival timing or packet loss has an effect on facsimile communication. Facsimile communication is a communication method which executes a repetition of procedure transfer and image data transfer while establishing synchronization between the modem on the sending side and the modem on the receiving side. In a communication method in which such synchronization between the sending side and receiving side is necessary, a lack of a communication signal due to the instantaneous interruption causes loss of modem synchronization, and makes it difficult to maintain a procedure signal transmission state or continue communication of image data. In particular, when executing facsimile communication on a modem complying with ITU-T Recommendations V.34, occurrence of the instantaneous interruption has a large effect.
FIG. 3 is a diagram describing a time sequence of a process in which a facsimile communication ends with an error on the receiving side from the appearance of sending/receiving signals in the case of occurrence of instantaneous interruption of an audio signal due to an effect from an IP network during communication with a V.34 modem.
Because image data of V.34 communication is composed of continuous frames, if an instantaneous interruption occurs during reception of an image signal, image data of a frame cannot be received during the time from loss of synchronization of frame communication until resynchronization. Also, processing for resynchronization is complicated, and if an instantaneous interruption of a line signal longer than a certain amount of time period occurs, resynchronization is impossible, and as the modems diverge, normal reception of image data becomes impossible.
In order to solve such a problem, Japanese Patent Laid-Open No. 04-255166 discloses technology to, if an error frame is detected and a valid frame cannot be received within a certain time period until the next correct frame is received, shift to a low-speed procedure and receive a low-speed command from the other side.
However, in the conventional technology, if an instantaneous interruption occurs again when an error frame is being resent, the modems diverge, and it becomes impossible to recognize the rear end of a primary channel on the receiving side. The rear end of a primary channel indicates a control signal for shift a communication channel from a primary channel for executing reception of image data to a control channel for executing transmission/reception of a sequence signal. In such a case, when the receiving side determines that a certain amount of time period has passed in a state in which the rear end of a primary channel has not been detected, a timeout error occurs and the process terminates with an error.